Workshop Techniques - Telescopic Gauges - Model Engineer

A bit of a personal history for starters:

Goto RTP GAUGE to know more.

My sandwich course, -, included twelve months factory attachment of which the first few months were spent in the Trainee Modelshop. (Question #1: why was it called a 'modelshop' ? ) I made some BA spanners and a Browne & Sharpe toolpost spanner, some toolmakers' clamps and a scriber and centre punch or two. They can't have turned out too badly – one of the toolmakers' clamps was nicked!

Subsequently, such workshop skills as I have, have been built up, patchwork fashion, relying on books, live examples and, recently, YouTube videos and lots of trial and error along the way.

One blank area in the patchwork has been hands-on experience with using the telescopic gauge to measure bores. I've never tried it.

I've watched the famous YouTube gurus inserting the telescopic gauge into the bore with a forward lean, pulling it past 'top dead centre' and then measuring it with a micrometer. I presume that the gauge is set with just sufficient tightness to hold the moving arm still but not too much to prevent the arm moving within its mate to adopt the diameter of the bore.

But, question #2, do they then tighten the handle to prevent the closing force of the micrometer from altering the reading?

Question #3, how is tthe technique altered if the depth of the bore is, say, only just sufficient to admit the head of the gauge?

I know that attaining this sort of skill is best achieved with the hands-on aid of a mentor; mentor performs operation, learner performs operation, sequence repeats until both achieve the same reading. I don't have that opportunity but I do have to machine a spigot to fit a recess (think chuck + backplate) and there'll be no second chance. I have mounted chucks on backplates in the past by just very carefully taking off less and less until they fit. I'd like to have a bit more control this time.

I look forward to members' comments. How do you do it?

Bestt regards,

Swarf, Mostly!

P.S.: I do have a digital caliper gauge.

I find they are fairly accurate, +-0.001" mostly, but they vary in quality which can affect their "feel". If I can, I use the Moore & Wright ones, they are smoother than my cheap set. They can be dismantled and the 45 degree ends made smoother, many people have done this mod.

My standard method is to compress and lock the gauge before putting it in the hole. Then unlock with the gauge handle tilted relative to the axis of the bore and then lock just enough to hold the setting. Then rock the gauge (I prefer having the ends in a vertical plane if possible) past the tightest point and remove. Careful measuring without changing the size last. Repeat for critical measurements to prove matching figures.

Practice on known bore sizes if possible, I have a ring gauge, but with care a locked micrometer can be used, but you need to be square each time with parallel faces.

I do as others have said rocking the lightly nipped up gauge across the diameter once to set the guage. I always tighten it slightly before measuring.

When measuring I support one end of the gauge on the micrometer anvil then rock the gauge slightly as the mic is closed to feel for contact as it passes over centre so to speak. Always repeat several times to get a mean but never rock across the bore and back to set it – just the once only

Neil's tip of using a bearing is a good one – not heard that before – but a good way to establish to yourself you are getting the right kind of feel and measurement skills from an exact known source.

Tug

And on holes 1/2" and under you are better off using ball gauges. Much easier to set, giving more reliable result for the inexperienced (and the out-of-practice experienced too!).

I usually rock T gauges in two planes. First — with the T at an angle — side to side to make sure the T gauge is sitting dead centre in the circle of the hole, then carefully rock it over centre lengthways once to get the size. Same technique in the mike to get careful measurement without moving the plunger of the T gauge.

It does take practice and "feel" and if you are not doing it all the time, you do get rusty. These days, proper inside mikes and dedicated bore gauges are not that expensive so are worth looking into.

Edited By Hopper on 29/08/ 10:13:09

Posted by Circlip on 29/08/ 11:32:19:

Ask ten 'Engineers' the same question and you'll get ten different answers, the same applies to 'Feel'. Some rely on a certain number of clicks of the ratchet stop but that doesn't allow for the speed of spinning the spindle closed. Doubt with his skills Ramon would really need the extra 'nip' to ensure not pushing the bar back. Wonder what the blind machinists (NO slight OR pun) comparison measurements are using a Braille mic? The ability to get a 25mm measurement on a 25.1mm block is extremely easy for some and especially winding the toolroom foreman up "Can i borrow the 1-2" adjustable spanner.

Feel is an acquired thing that applies when fitting parts together and don't even start to introduce tolerances.

Regards Ian.

Good afternoon, Ian,

Thank you for your post. (Thank you to all other posters as well but I want to reply particularly to Ian. )

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I started this thread with some trepidation because I do appreciate that 'feel' is not easy to communicate using the written word. For that reason, I am grateful to everyone who has bravely made a contribution.

However, my opening post does contain three specific questions. #1 is a matter of interest but is a side issue from my main enquiry. Several contributors have answered #2 but nobody has addressed #3. The task I have before me does involve a recess that is too shallow to permit 'rocking ' of the telescopic gauge.

Thank you again to everyone who has commented and I look forward to still more contributions.

Best regards,

Swarf, Mostly!

Sorry for the omission SM.

Shallow depth bores are difficult at best. When boring out the bearing housings on the IC engines that very situation arises Though I 'size' the hole with a accurately turned plug gauge I do use the tele-gauges to get me close.

In this case I try to hold the gauge as carefully as possible across the diameter of the bore with it just loose then gently nip up. i.e. no rocking though centre. This can sometimes give a false reading due to potential movement, albeit very slight, on the tightening action but several readings will get you pretty close.

I had a set of Moore and Wright gauges and later bought the double sided versions – Draper I think. They were as someone said a bit hit and miss and 'draggy' but an hour or so taking them apart and stoning all sliding surfaces transformed their use. I sold the M&W ones on.

Best – Tug

Swarf Mostly, re your #3

I'm not sure what you mean by Digital Calipers on your p.s. in the first post.

I always struggle a bit with smaller internal measurements, up to a couple of inches or so; or at least I always think I do, so resort to trying to take measurements by two different methods and comparing them.

I'm fortunate in having all sorts of measuring instruments that I've picked up cheaply over the last 30-40 years, several of which are intended for internal distances.

If your Digital Calipers, are of the sliding 6" scale type, like vernier calipers, but digital, then yes I have them in vernier, digital and dial varieties, as well as conventional internal micrometers, and a 3 point bore gauge.

By far the easiest instrument I have for bore/internal measurements are a couple of sets of MPJ dial calipers, which came via ebay: I most commonly use a set which covers about ½" to 1¼"
I think I paid about £20 delivered, but they regularly show up in the £30-40 price range.

They are similar in style to these eye wateringly expensive Mitutoyo ones from Machine DRO
https://www.machine-dro.co.uk/mitutoyo-internal-dial-caliper-0-4-1-2-0--209-352

Even though mine are old, they are easy to use and repeatable to less than a thou with care, and ideal for shallow bores, short linear steps etc.

Bill

Ref #3 from OP For shallow bores and grooves, like Bill I use Dial calipers and a mic to check over the ends when the dial reads as the bore/groove indicated.
Sometimes it's better to set the mic to the dimension required and check the reading on the caliper dial so you have a target to get to.

As Bill said these do come up for sale fairly often and are well worth the investment as they are also useful to check any bore is parallel.

For smaller than 12mm diameter holes/bores I find ball type gauges and a mic give accurate readings but only with practise as is the case with telescopic gauges.

Several other useful methods for measuring bores can be seen in my Photo Album. (Internal Measuring Devices)
https://www.model-engineer.co.uk/albums/member_album.asp?a=

Emgee

Edited By Emgee on 30/08/ 11:42:00

Posted by roy entwistle on 29/08/ 12:33:58:

How many people use calipers these days. When I started my apprenticeship in I was expected to have inside and outside calipers, a square and a six inch rule, and know how to use them. We were expected to have our own tools because it taught us to respect them. You never borrowed tools off another workman. Tools could be bought at the works shop, at a discount and paid for weekly.

i still use them from time to time. (The old spring calipers, not vernier or digital.) Actually handy for the question in hand about measuring the diameter of a shallow recess, then measure the calipers with a micrometer.

We were issued with them as apprentices in the 70s but I rarely remember ever using them much. But prior to about WW2 or a bit after, much reasonably precision work was done with calipers and six inch rule, as you say. Final fit and finish would be done by direct comparison with the mating part. I know they were my old man's go-to measuring tool on the lathe. He was an apprentice of the s.

Hi,

It seems that a good quality inside caliper should be capable of making the same measurements to the same type of accuracy as a set of good telescoping gauges. So - why would I buy a set of telescoping gauges instead of a good inside caliper that can handle all of the hole sizes the set would?

Thanks!

I think we all know what telescoping gauges are but not so sure about inside calipers. The picture is what I call an inside caliper.

Which is better/more accurate? It all depends on use. To use telescoping gauges you also need a micrometer, in fact 6 if you want to cover the range up to 6"/150mm. Takes longer time too to measure with telescoping gauges.

Personally, when I use a caliper (inside or standard) to measure internally, the first thing I do is to calibrate it against a ring with a known ID. Closing it and zeroing it just isn't good enough. I zero on the known hole diameter and add that onto the measurement result I get from the hole measurement. I'd feel confident of getting a result within 0.02mm/0.001". Maybe even less.

As to telescoping gauges then it'll very much depend on who's using them and their experience with them.

Now if what I've shown isn't what you mean by "inside caliper" then you'd better tell us what you do mean.

Hi,

It seems that a good quality inside caliper should be capable of making the same measurements to the same type of accuracy as a set of good telescoping gauges. So - why would I buy a set of telescoping gauges instead of a good inside caliper that can handle all of the hole sizes the set would?

Thanks!

First reason: "reach". Second reason: "minimum size". Third reason: "accuracy". Fourth reason: Odd shape of a bore and/or its SUB features to be measured.

One has to have "all of".. gage pins, telescoping gages, inside mics, inside calipers, bore gages, and now and then even spring calipers to get around obstructions or Indicals or similar goods for internal grooves.

Each have advantages and disadvantages.

Like it or not, if you can only have ONE, it would be the ancient spring caliper.. and good OUTSIDE mics with reliable setting standards to transfer to/from. PITA, but the most universally usable.

Tedious - DAMHIKT! It is actually all I DID have for a short while.

But VERY short, that while was to be .... before I started adding.. "all of the above".

Inside thread mics still "to do", but not much else has been missed-out. The design of the spring calipers goes back to when a shaft and bore were made to fit each other- not made to any measured standard. If you have enough time to make repeated adjustments and measurements, and develop a good "feel" for them I suspect you could get by with them in a home shop. But getting by is all you will be doing, and wasting time doing it.

Measuring tools is not the place to try and save money. They will cost you in time and material, lead to frustration, and suck the joy out of you. Rather than answer your question directly. I'll tell you how to measure anything. Never do it once. Always do it multiple times until your repeatability gives you confidence that your reading is correct. That is the ONLY correct procedure. Now, whatever tool provides you with that level of confidence is the right one. The rest is personal preference. Remember, these tools were made to satisfy a requirement. Just because you have not found the need for the tool does not mean the requirement doesn't exist. It just means you have not been in the trade long enough.

Rather than answer your question directly. I'll tell you how to measure anything. Never do it once. Always do it multiple times until your repeatability gives you confidence that your reading is correct. That is the ONLY correct procedure. Now, whatever tool provides you with that level of confidence is the right one. The rest is personal preference. Remember, these tools were made to satisfy a requirement. Just because you have not found the need for the tool does not mean the requirement doesn't exist. It just means you have not been in the trade long enough.

It's not that I disagree with you so much as I'd probably express it differently.

Measuring multiple times and getting the same result only means you are consistent. It doesn't mean you are getting a correct measurement result.

Probably the same as calibrating anything and using something you know the exact measurement of. As long as there is user influence involved you'll go for the "desired" measurement.

Rarely done but much better to measure an "unknown" dimension, write down what you get and then measure with something you know will give you an accurate result. First then will you know how good you are and how good what you intend using is.

When I claim to measure this and that and state the accuracy I get it's never done based on using just one measuring device. When I read in PM now and then how accurately some say they measure I often wonder if it is accuracy or repeatability they've achieved. Of course without repeatability no accuracy.

It might just be said I'm calibrating myself too

Hi,

It seems that a good quality inside caliper should be capable of making the same measurements to the same type of accuracy as a set of good telescoping gauges. So - why would I buy a set of telescoping gauges instead of a good inside caliper that can handle all of the hole sizes the set would?

Thanks!

I don't think calipers are as accurate as telescoping gauges measured with a good set of mics. As with anything else using them takes practice. I took some master set rings and measured them over and over to have a comparison between what I measured them at to the actual size of the ring. It's how much you tighten the lock screw that causes the most variation in measurements. I think you should buy both, I did and haven't regretted it.

Brent

I don't think calipers are as accurate as telescoping gauges measured with a good set of mics. As with anything else using them takes practice. I took some master set rings and measured them over and over to have a comparison between what I measured them at to the actual size of the ring. It's how much you tighten the lock screw that causes the most variation in measurements. I think you should buy both, I did and haven't regretted it.

Brent

.
40 years machining and maybe used telescoping gages for maybe 2 hours compared to using indicating bore gages maybe hours
.
just saying indicating bore gages will easy see taper and out of round literally in seconds. never seen a professional even bother with telescoping gages.
.
and obviously 99.999% of professional wouldnt bother with the caliper either unless tolerance was over .002" , sure you occasionally find the guy who says he can measure .001" with his $20. calipers....... obviously he aint making many expensive parts. if you got $20,000 part with a oversize bore it can be a very expensive rework job

Hi,

It seems that a good quality inside caliper should be capable of making the same measurements to the same type of accuracy as a set of good telescoping gauges. So - why would I buy a set of telescoping gauges instead of a good inside caliper that can handle all of the hole sizes the set would?

Thanks!

Simple mechanics.
An inside mic, or telescope gauge detect contact in a single 'line' or axis of the item. That includes whatever tolerance or imperfections are contained in the tool. A good hand should/ could get near identical readings blindfolded.

A caliper has contact points far from the frame they slide on, which requires clearance to move. The only plus to calipers is quickness, not accuracy, and can be compact compared to same size micrometer. Why digital calipers display . is beyond me, other than interpreting something different than what they display seconds before. That is neither true resolution or accuracy. Regarding bores or OD's, those are always subject to interpreting squareness to axis being checked. Try that blindfolded?

.
40 years machining and maybe used telescoping gages for maybe 2 hours compared to using indicating bore gages maybe hours
.
just saying indicating bore gages will easy see taper and out of round literally in seconds. never seen a professional even bother with telescoping gages.
.
and obviously 99.999% of professional wouldnt bother with the caliper either unless tolerance was over .002" , sure you occasionally find the guy who says he can measure .001" with his $20. calipers....... obviously he aint making many expensive parts. if you got $20,000 part with a oversize bore it can be a very expensive rework job

I worked in an automotive machine shop, so a lot of the work was ‘boring’. Usually the head machinist would handle that task, however in order to aid in my learning I was encouraged to measure and check bores.

Often enough we used telescoping gages. An indicating bore gage doesn’t fit inside of something like a differential housing. For bores that had easy access of course we used an indicating bore gage because as you note, it’s easier and offers the ability to check for ovalocity.

I don’t recall ever using an inside mic, I don’t recall if that was the machinist’s preference or if there were further logic to it.

We did use calipers, mainly just for ‘ball-parking’, never for precise work.

Anywho, I mention this simply to ask what you typically measure with when the part is on the machine? I don’t always, in fact rarely have access enough to a bore to check it with an indicating bore gage, but often enough I do have room for snap gages. This is a relatively old thread revived again. I suppose I'll never understand why calipers (and I personally prefer digital) seem to create so much difference of opinion.

I can't think of a more inexpensive, versatile measuring tool. Outside, inside and depth. As far as accuracy goes then it's a question of what you use it for. If a standard 150mm / 6" digital caliper isn't accurate when measuring external to within 0.03mm / " then you have a defect caliper.

Feel is very important and measuring blindfolded is ridiculous. I check my calipers now and then against something I know the measurement of. Small ball bearings are handy to have.

I have the feeling many still have old calipers or buy second hand. I'm lucky in that mine are all relatively new. Of course I have and use micrometers (outside and inside) and dial indicators. It all comes down to what I want to measure and how accurately I need to measure.

YouTube

I'll also just add:

A VERY MERRY CHRISTMAS TO ONE AND ALL.

This is a relatively old thread revived again. I suppose I'll never understand why calipers (and I personally prefer digital) seem to create so much difference of opinion.

I can't think of a more inexpensive, versatile measuring tool. Outside, inside and depth. As far as accuracy goes then it's a question of what you use it for. If a standard 150mm / 6" digital caliper isn't accurate when measuring external to within 0.03mm / " then you have a defect caliper.

Feel is very important and measuring blindfolded is ridiculous. I check my calipers now and then against something I know the measurement of. Small ball bearings are handy to have.

I have the feeling many still have old calipers or buy second hand. I'm lucky in that mine are all relatively new. Of course I have and use micrometers (outside and inside) and dial indicators. It all comes down to what I want to measure and how accurately I need to measure.

YouTube

I'll also just add:

A VERY MERRY CHRISTMAS TO ONE AND ALL.

I’m not sure if this directed at my comment. We used high quality calipers, not some second hand stuff. Currently, I use mine a lot so I‘ve performed all of the certification procedures on my own calipers. I know they’re accurate to .001” in almost all circumstances according to gage blocks.

However most of the boring jobs were to house a bearing or to retain a sleeve and so a higher degree of precision provided a measure of confidence.

I’m not sure if this directed at my comment. We used high quality calipers, not some second hand stuff. Currently, I use mine a lot so I‘ve performed all of the certification procedures on my own calipers. I know they’re accurate to .001” in almost all circumstances according to gage blocks.

However most of the boring jobs were to house a bearing or to retain a sleeve and so a higher degree of precision provided a measure of confidence.

My post wasn't directed at you. Just general. As you say, to house a bearing requires more accuracy than a caliper is capable of.

To all.
To me there are 4 types of measurement caliper. Vernier, dial and digital. The 4th ("leg") is the type that requires another measuring instrument to "calibrate" the result.

Might be good if what type is meant when "caliper" is mentioned.